David Cameron has started the countdown for British military action in Syria, with a Commons vote on Wednesday authorising RAF air strikes on the Islamic State (IS) militants in their heartlands.

The Prime Minister's decision to go to the House came after a tumultuous meeting of the shadow cabinet agreed that Labour MPs should be given a free vote - something party leader Jeremy Corbyn had previously opposed.

Returning to Downing Street after attending the opening of international climate change talks in Paris, Mr Cameron said there was "growing support" across Parliament to intervene against IS - also referred as Isil, Isis or Daesh - in Syria.

"It is in the national interest, it is the right thing to do, we will be acting with our allies, we will be careful and responsible as we do so, but in my view it s the right thing to do this to keep our country safe," he said.

Mr Cameron rejected a call by Mr Corbyn for a two-day Commons debate - which would have meant delaying the vote until next week - although he made clear he would allow extended time for discussion on Wednesday.

Foreign Secretary Philip Hammond indicated that, if MPs voted for air strikes, more war planes would be deployed to the region to support the RAF Tornado GR4s already attack IS in Iraq.

Labour sources suggested that 43% of Labour MPs - almost 100 out of the party's tally of 231 - supported air strikes in Syria, although it was unclear how many will vote with the Government.

Nevertheless, Mr Cameron - who has always said he will only go back to Parliament if he was sure of winning - clearly now believes that he has the numbers in the bag to offset any Tory rebels still opposed to intervening.

On the Labour side there was widespread dismay at Mr Corbyn's handling of the issue, following his statement last week that he could not support air strikes - seen by critics as an attempt to pre-empt the shadow cabinet meeting.

Even before the Labour top team gathered at Westminster, Diane Abbott - one of Mr Corbyn's few shadow cabinet allies - was warning that it would be a mistake to give Labour MPs a free vote and that they should be ordered to oppose air strikes.

"The problem about a free vote is it hands victory to Cameron on these air strikes, it hands victory to him on a plate. I don't think that's what party members want to see," she told the BBC Radio 4 Today programme.

Before the meeting began, Labour sources released analysis of emails received over the weekend from party supporters which they aid showed 75% in opposed to military action.

But with support for air strikes headed by shadow foreign secretary Hilary Benn and deputy leader Tom Watson - and even leadership loyalist John McDonnell calling for a free vote - Mr Corbyn appears to have had no choice but to back down.

Mr Corbyn will now open the debate for Labour while Mr Benn will do the wind-up - although they will speak on opposite sides of the argument.

A senior acknowledged the arrangement was unusual but insisted that it was not "shambolic".

A spokesman for Mr Corbyn said that Mr Cameron's refusal to allow a two-day debate showed his case for war was "falling apart".

"Matters of national security are far too important to be bulldozed through the House of Commons for political convenience," the spokesman said.

Some Labour MPs questioned whether there would be a genuinely free vote, with John Woodcock, who supports military action, complaining of "certain people floating around the edges" warning of the "dire consequences" for MPs political futures if they went against Mr Corbyn.

The Labour leader faced further criticism from backbenchers at the weekly meeting of the Parliamentary Labour Party at Westminster.

Veteran David Winnick was said by one senior figure present to have denounced Momentum - the pro-Corbyn, hard-left pressure group - as a "party within a party" and said attempts to "shake down" MPs with the threat of de-selection were "completely unacceptable".